The use of tetrabasic lead sulfate, chemical formula 4PbO.PbSO.sub.4, has been determined to be a useful starting material for the preparation of positive electrodes for use in lead-acid batteries. As disclosed in U.S. Pat. No. 3,765,943, issued to Biagetti on Dec. 9, 1970, entitled "FABRICATION OF LEAD-ACID BATTERIES", and in U.S. Pat. No. 3,899,349, issued to Yarnell on Aug. 12, 1975, entitled "CARBON DIOXIDE CURING OF LEAD-ACID BATTERIES", hereby incorporated by reference, battery plates or electrodes fabricated with tetrabasic lead sulfate and cured in a particular manner in the presence of a controlled amount of carbon dioxide gas, produces an increased product yield, greater life, greater reproducibility in the chemical and physical properties of the product, and crystal morphology more suitable to battery operation. It should be noted that both positive and negative electrodes may be formed with tetrabasic lead sulfate, although it is preferred that tetrabasic lead sulfate be used only for positive plates. In forming or charging a battery, the desired result is a positive plate having lead dioxide as the active material, and a negative plate or electrode having metallic lead as the active material. It has been found, for negative plates and electrodes, more practical to start with a mixture of lead oxides containing approximately 30% metallic lead particles.
The preparation of active electrodes or plates for lead acid batteries involves three steps. First, a paste is applied to a supporting grid, the pasted grid is dried and cured, and the cured grid is formed to produce the active materials used in the charged lead-acid battery. Formation is usually by electrolyte formation, by passing current through the electrode, the desired result being a positive plate having pellets of lead dioxide, and a negative plate having pellets of metallic lead.
Tetrabasic lead sulfate, unlike prior materials used for pasting battery electrodes or plates, is non-reactive. It contains no acid, and is not self-hardening.
It has been stated that the only parameters relevant to paste mixing are the amounts of tetrabasic lead sulfate and water, and the length of time mixed, by C. F. Yarnell, abstract No. 32, issued Oct., 1974, New York, Electrochemical Society Meeting.
When the water is first added the mix appears very dry. After continued mixing, the mixture starts to form into small balls, these small balls coalescing into larger balls until the mixture becomes a single mass. As the paste is mixed beyond this point, it becomes more fluid, eventually becoming too fluid to use, regardless of the amount of water used to make the paste, as long as a minimum amount is used.
The paste is then applied to an electrode, typically in the form a gridwork formed of metallic lead or a lead alloy, and dried and hardened.
The overall problem is maintaining sufficient mechanical strength in the pasted electrodes to survive handling and forming. Plates may become damaged either by accidental rough handling or may tend to disintegrate, either by popping pellets of material, or by shedding material while being formed, typically in a dilute sulfuric acid electrolyte. If this occurs, in initial forming, or in charging in use, battery performance and life expectancy is diminished, the loose material forming shorts between plates, either directly, or as it builds up on the bottom of a battery, and the electrode becomes more porous, having poor contact with the underlying grid.
After drying, the prior art hardening process for plates or electrodes formed from tetrabasic lead sulfate paste requires carbon dioxide and water. It is essential to such a procedure that the pasted electrode be exposed to an atmosphere of carbon dioxide while water in the form of water vapor, of at least 35 percent relative humidity, is in contact with the paste material.
A preferred prior method includes drying the plates or electrodes, then curing them in an atmosphere of 50 percent relative humidity containing 20 percent carbon dioxide gas.